1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by using detection of reflected or transmitted electromagnetic (EM) wave obtained by irradiation of a target object with electromagnetic wave, and particularly to an image forming apparatus using EM-wave in a frequency range between about 30 GHz and about 30 THz. In this specification, terahertz (THz) wave or millimeter wave is used for electromagnetic wave in the frequency range between about 30 GHz and about 30 THz.
2. Description of the Related Art
In recent years, the image forming having been widely performed in a light frequency range is also considered to be important in a THz wave frequency range. The reason therefore is that the transmissivity of THz wave for certain objects is relatively strong, different from light. This characteristic can be employed in a useful way.
In the THz wave frequency range, however, the speed of development of devices, such as a THz wave generator and a THz wave detector, is relatively slow. Accordingly, the following methods have been typically used for the image forming in the THz wave frequency range.
In one method, electromagnetic wave in the THz-wave frequency range is condensed into a point in an object to be imaged, and the point is moved relatively to the object to obtain the image thereof. This method is different from a method of obtaining a two-dimensional image at a time, and does not necessarily require a strong irradiation with the THz wave generator and a highly sensitive detection with the THz wave detector. Therefore, this method is frequently used.
JP 08-320254 A discloses the above image forming method. In a method disclosed therein, electromagnetic wave is condensed into a certain area of the object, the object is moved relatively to the condensed electromagnetic wave, and the condensed electromagnetic wave is caused to pass plural areas of the object. Transmission conditions of the electromagnetic wave through the plural areas are detected, and the detection signals are processed by an image forming unit or portion to acquire an image of the object. The condensation is executed since a signal-to-noise ratio (SN ratio) between detection signal and noise is low. The SN ratio is increased by the condensation of the THz wave.
When the state of the THz-wave condensation onto a certain area of the object is caused to last for a given period of time to accumulate plural detection signals, the SN ratio is further increased by the effect of averaging the accumulated signals. Accordingly, the image is typically formed in such a manner.
In the above method, however, it takes a considerable time to accumulate the plural detection signals. Therefore, there is the relationship of tradeoff between the improvement of SN ratio and the time required to form the image.
The tradeoff relationship also exists in a time-resolved spectroscopy called a terahertz time domain spectroscopy (THz-TDS) that differs from the image forming apparatus. Occurrence of the tradeoff relationship is due to performance of time resolution of a pulse waveform emitted from a photoconductive switch in the THz-TDS. When obtaining the pulse waveform, the time resolution should be conducted for a point of the pulse waveform while accumulating plural values to improve the SN ratio. Such operation leads to the occurrence of the tradeoff relationship.
To solve the above disadvantage, WO 00/079248 A1 proposes a method in which the improvement of SN ratio and a decrease in the time required obtain the pulse waveform are achieved by using a process of periodically obtaining pulse waveforms. For this purpose, delay time for a point of the pulse waveform is periodically changed. With the pulse waveform thus obtained, the bandwidth can be restricted by a spectrum analyzer or the like, and the SN ration can be improved.
However, though the determination of restriction of the bandwidth is relatively easy in the above method using the process of periodically obtaining pulse waveforms, the determination of restriction of the bandwidth is not easy in forming the image. While the bandwidth for the pulse waveform can be predicted in advance, the bandwidth of a transmissivity distribution or reflectance distribution of the object to be imaged generally depends on each individual object, and cannot be predicted in advance. Therefore, in the field of the image forming, there exists a demand for achieving both the improvement of SN ratio and a decrease in the time required to form the image.